Numerous types of rail car brake systems are in use today. Some are pneumatically operated; some, electrically; others, electro-pneumatically. While various nationally recognized standards ensure that rail cars of different carriers are compatible with each other in most respects so that trains may be made up from cars of several carriers, there are some brake systems which provide features not really compatible with the systems of other rail cars. Of course, the differences may be desirable to suit some specific requirement of an individual carrier; but, nonetheless, to the extent a rail car is incompatible with other cars in use, its applications become limited.
There are two basic types of brake control valves in general use today. The first is the "direct release" control valve which applies the brakes in proportion to the reduction of brake pipe pressure and releases the brakes directly on any increase in brake pipe pressure. The second is the "graduated release" type valve commonly used in passenger trains which applies the brakes in proportion to brake pipe pressure reduction, but releases the brakes in proportion to the restoration of brake pipe pressure.
In service then, the graduated release valve permits smoother control of a train by permitting an engine man to partially release brakes. In contrast, partial release of brakes with a direct release valve is impossible; and if an engine man determines that he is stopping too fast, he must completely release the brake, and then re-apply it.
The prior art discloses an electro-pneumatic system for effecting a graduated release of brakes with a direct release control valve by placing a "holding magnet valve" in the exhaust pipe from the control valve, and energizing or de-energizing this magnet valve by response to manual manipulation of the engine man's brake valve so as to prevent or permit exhaust from the control valve even though this valve is, at all times, in its "release" configuration.
Difficulties arise when cars having graduated release control valves are to be coupled with cars having the electric holding valve type of graduated release. If cars equipped with graduated release type control valves continue to provide graduated release while those equipped with the holding magnet valve and direct release control valve do not, due to the absence of their usual controller, the undesirable condition arises where the brakes on some cars will be fully released while those on others will be still applied. Those skilled in the art will appreciate that under various operating conditions, this can produce discomfort to passengers because of slack action as the unbraked cars try to move toward or away from the braked cars to which they are coupled. Overheating of the wheels of the braked cars may also result.